The present invention relates to a method and a device for controlling electrically actuated weft brakes for the automatic adjustment of the mechanical tension of a weft thread in textile machines with mechanical insertion; this expression is used to designate textile machines such as gripper looms, bullet looms and picking-type machines in which the weft thread is engaged mechanically by transport elements in order to be inserted in the shed; fluid-jet weft insertion machines are therefore excluded.
As it is known, in modern weaving systems the weft thread that unwinds from the spool is fed to the loom by means of a weft feeder. Such weft feeder is an apparatus which comprises a fixed drum on which a rotating arm winds and restores a plurality of turns of thread which constitute a weft reserve and from which the turns unwind when requested by the loom, at each beat thereof.
Electrically-actuated weft braking devices are inserted between the feeder and the loom and have the dual purpose of maintaining on the thread a sufficiently high mechanical tension in certain critical steps of the weft insertion process and of ensuring that in any case, throughout the insertion process, the instantaneous or peak value of the mechanical tension does not reach excessively high values such as to break the thread, for example at defective points of the thread. The critical steps of the insertion process in which correct control of weft thread tension is indispensable are constituted, e.g. in a gripper loom, by the thread gripping step, by the step for transfer between the retaining gripper and the drawing gripper, and by the step for the arrival of the weft thread.
In order to achieve such dual purpose, the use of weft braking means which are integrated in the feeder and/or separated from it is known.
The present description non-limitatively relates to the latter means, i.e. the so-called duckbill brakes arranged downstream of the feeder and comprising a pair of mutually opposite laminas, respectively a fixed one and a movable one, between which the thread runs and in which an electromechanical actuator actuates the movable lamina so as to press more or less intensely on the fixed one in order to vary the applied braking action. As it is also known, the actuator of the brake is driven with an excitation current modulated by a tension sensor which is inserted between the weft brake and the weaving loom and generates a modulation signal for the excitation current which is proportional to the instantaneous value of the mechanical tension of the weft thread.
More specifically, weft thread tension adjustment systems are known which use, in order to control the weft brake, an adjustment loop in which the reference is constituted by a preset and selected value of the mechanical tension of the weft thread and in which the instantaneous and actual value of the tension, measured by a tension sensor, is subtracted from the reference value in order to obtain an error signal. The error signal is processed by logic means (logic block of the PID type), which obtain an information signal capable of eliminating the error.
This information signal is matched, by means of adequate power circuits, by a proportional braking action applied by the laminar brake to the weft thread; the information signal is therefore termed "brake reference" hereinafter.
This conventional weft brake adjustment and control system, based exclusively on the direct measurement of the mechanical tension that affects the weft thread, despite being widely used, does not fully meet the requirements of modem weaving processes, since it is unable to compensate, sufficiently to avoid unwanted breakages of the weft thread, the rapid variations in mechanical tension to which the thread is subjected during the insertion process.
This severe drawback is substantially due to the fact that the braking action applied by the laminar brake or by other conventional types of driven brakes is heavily influenced by the travel speed of the thread, so that the mechanical tension generated on the weft thread, for an equal braking action applied by the brake, is just as directly dependent on the travel speed.
It is thus evident that the conventional above-cited adjustment systems are entirely insufficient and inadequate in weaving processes with mechanical insertion in which insertion speeds on the order of 2500 or 3000 meters/minute are easily reached and exceeded, and in which, during the insertion steps, the weft is subjected to the intense accelerations and decelerations that characterize the rules of motion of the thread transport elements.
In particular in the case of gripper looms, with reference to the angular positions of the driving shaft of the machine and as shown in the solid-line chart line of FIG. 2, the step for gripping the weft thread occurs at approximately 65 shaft degrees. After this, the thread is subjected to a step of transport with a high initial acceleration up to approximately 120 shaft degrees, followed by a deceleration, due to braking, until approximately 180 shaft degrees are reached, this being the position for transfer between the retaining gripper and the drawing gripper. Such drawing gripper, over the remaining arc of the insertion step, subjects the thread to an acceleration-deceleration cycle which is substantially identical to the preceding one, ending insertion at approximately 287 shaft degrees.
Correspondingly, the mechanical tension on the weft thread varies substantially according to the same rule of variation as the thread travel speed and therefore has the plot of the dashed chart line of FIG. 2, which is also substantially characterized by two positive half-waves having a minimum cusp which lies substantially at the 180 shaft degrees position; such minimum value, different from zero, corresponds to the value of the static tension that acts on the thread at rest.